1. Field of the Invention
The present invention relates to a method for producing a grain-oriented electrical steel sheet having an improved watt loss-characteristic and a high magnetic flux density, and used for the core materials of a transformer or the like.
2. Description of the Related Art
A grain-oriented electrical steel sheet is a soft magnetic material used as the core materials of mainly, a transformer or other appliances, and should have good exciting and watt loss-characteristics.
The exciting characteristic is numerically expressed by B.sub.8 (the magnetic flux density at an 800 A/m intensity of the magnetic field). The watt loss characteristic is numerically expressed by W.sub.17/50 (watt loss per 1 kg when magnetized at 50 Hz up to 1.7 T).
The grain-oriented electrical steel sheet is obtained for developing usually by utilizing the secondary recrystallization the so called Goss texture having {110}plane on the surface of a steel sheet and &lt;001&gt;axis in the rolling direction. To obtain good magnetic properties, it is important to precisely align the &lt;001&gt;axis, which is an easy direction of magnetization, in the rolling direction. The magnetic properties are greatly influenced by sheet thickness, grain size, resistivity, surface coating, purity of a steel sheet, and the like.
The orientation property has been drastically enhanced by methods which are characterized by using MnS and AlN as the inhibitors and a heavy, final cold-rolling. Together with the enhancement in the orientation property, the watt loss characteristic has been also considerably enhanced.
Meanswhile, under the background of recent increases in energy costs, the transformer producers have further intensified their tendency to use low watt loss-blank materials. Although the development of amorphous alloys, 6.5% Si steel and the like has advanced, there still remain problems in the industrial use of these alloys for transformers. On the other hand, the techniques of controlling magnetic domains by a laser and the like have been recently developed and have drastically improved the watt loss characteristic. In addition, since the effect of the technique of controlling magnetic domains becomes higher when the product sheet thickness is thinner and the magnetic flux density is higher, there is an increasing necessity to develop products having a thin sheet thickness and a high magnetic flux density.
A method is known for enhancing the magnetic flux density by using the AlN inhibitor and a heavy final cold-rolling at a rolling rate of more than 80%. This method, however, involves a problem of unstable secondary recrystallization at a thin sheet thickness.
U.S. Patent No. 3,632,456 proposes a method for solving this problem by annealing a hot-rolled strip, successivly cold-rolling and intermediate annealing, and subsequently, carrying out a heavy final cold-rolling at a draft exceeding 80%. The secondary recrystallization is stabilized at a thickness down to 0.14 mm by this method, but a completely satisfactory watt-loss characteristic is attained only with difficulty, because of, for example, a decrease in the magnetic flux density.
As described above, there are problems remaining in enlarging the range of a sheet thickness, in which products have an improved watt loss and high magnetic flux density are obtained, to include those having a thin sheet thickness.
Japanese Examined Patent Publication No. 54-13,846 discloses that, in the production of a grain-oriented electrical steel sheet having a high magnetic flux density by utilizing AlN as the inhibitor and carrying out a single heavy cold-rolling at a rolling rate of from 81 to 95%, the magnetic properties are improved by aging during the single heavy coldrolling. Further, Japanese Examined Patent Publication No. 56-3,892 discloses that, in a method for producing a grain-oriented electrical steel sheet by cold-rolling twice or more, the magnetic properties are improved by subjecting the steel to aging during the final cold-rolling and by controlling, in a relationship with this aging, the cooling speed of an intermediate annealing which is a step preceding the last final cold-rolling. It is also disclosed in Japanese Unexamined Patent Publication No. 58-25425 that, in a method for producing a grain-oriented electrical steel sheet by a double rolling method with a final cold-rolling rate of from 40 to 80%, the magnetic properties are improved by subjecting the steel to aging during the first cold-rolling and second cold-rolling. Nevertheless, these three techniques cannot provide products having an improved watt loss and high magnetic flux density, even for products having a sheet thickness of 0.20 mm or less.